The goal of our studies of T lymphocytes is to identify novel and functionally important proteins (enzymes, receptors) and to find their specific inhibitors, agonists, and antagonists. These studies are expected to lead to the development of new immunomodulating drugs. We earlier proposed the role of extracellular ATP and adenosine in thymocyte differentiation and in peripheral T-killer and T-helper cell effector functions. This, in turn, suggested the possibility of modulating T cell generation and functions using ATP and its analogs. Since the effects of ATP are determined by the levels of expression of its (purinergic) receptors, the focus of this project was to investigate the effects of steroid hormones and of T-cell antigen receptor (TCR)-crosslinking agents on the expression of p2 class of purinergic receptors. A transient and protein synthesis-independent enhancement of p2y2 receptors mRNA expression was detected in mouse thymocytes after the addition of steroid hormone in experiments designed to understand the physiological role of purinergic receptors. Control experiments excluded the possibility that increases in p2y2 receptors mRNA expression were due to the enrichment within the remaining steroid-resistant cells of a thymocyte subset with high constitutive p2y2 mRNA-levels. Triggering of T cell antigen receptor (TCR)-mediated intracellular signaling pathways in thymocytes through crosslinking of TCR by anti-TCR monoclonal antibody (mAb) or by addition of protein kinase C activator and Ca++ ionophore also resulted in the up-regulation of p2y2, but not p2x1 receptor mRNA. It is proposed that the rapid increase of p2y2 purinergic receptors mRNA expression could be a common early event in responses of T-cells to different activating stimuli. Taken together with the recently discovered ability of different purinergic receptor-mediated signaling to antagonize or enhance the effects of TCR-crosslinking and steroids on thymocytes, the rapid up-regulation of p2y mRNA may reflect an immediate early gene response in which the newly expressed cell surface purinergic receptors provide regulatory feed-back signaling from extracellular ATP and/or adenosine.